The present invention relates to an electronic watermark technique for embedding additional invisible information in digital motion pictures, and in particular, to a detection technique for which accumulation of signals is used to achieve detection providing a high level of reliability, even when the embedded signal is weak.
Generally, since the resolution of a motion picture is higher than that of a still picture, a signal embedded in a motion picture must be weaker than is a signal embedded in a still picture so that image quality can be maintained. Even if a strong signal is embedded, it is considerably attenuated after being subjected to a low-pass filter or to MPEG2 compression. Therefore, in a method for detecting signals independently in each frame, or a method for detecting signals in a fixed number of frames, since the strength of the signals to be measured depends on the compression process, the reliability of the detection is not always high. If a threshold value for enhancing the reliability is set, another problem arises that embedded signals can not be detected at all (false negatives).
A method is required whereby the reliability of the detection does not depend on the strength of a signal that is measured in a single frame.
However, means for resolving the problem are not taught in the xe2x80x9cdata hiding method and data extraction method employing statistical inspectionxe2x80x9d disclosed in Japanese Patent Application No. Hei 8-348426, the xe2x80x9cdata hiding method and system employing statistical propertyxe2x80x9d disclosed in Japanese Patent Application No. Hei 8-345568, and the xe2x80x9cmultiple frame data hiding method and detection methodxe2x80x9d disclosed in Japanese Patent Application No. Hei 9-88493.
Problems to be Solved by the Invention
It is, therefore, one object of the present invention to provide an electronic watermark method and system for which the detection reliability of embedded information does not depend on the strength of signals that are measured in frames.
It is another object of the present invention to provide an electronic watermark detection method and system for detecting embedded information based on a statistical observation value for each motion picture frame.
It is an additional object of the present invention to provide an electronic watermark detection method and system for changing a sign before accumulating a value obtained through observation of a motion picture frame.
It is a further object of the present invention to provide an electronic watermark detection method and system that can cancel a correlation (dependency) of sequential motion picture frames.
It is still another object of the present invention to provide an electronic watermark method and system for changing a sign of a signal to be embedded before information is embedded.
It is still an additional object of the present invention to provide an electronic watermark method and system for changing the signs of embedded signals at specific intervals.
It is a still further object of the present invention to provide an electronic watermark detection method and system for preventing an increase in a difference between sequential motion picture frames.
It is yet another object of the present invention to provide an effective method and system for a compression method, such as MPEG2 by using a difference.
It is yet an additional object of the present invention to provide a method and system with which synchronization, as it is related to signs, of an embedding process and a detection process is not required.
To achieve the above objects, information is prepared for embedding as a bit stream, the sign of the bit stream being changed in accordance with a sign inversion cycle, and the bit stream being embedded in a frame.